Chancroid, a ulcerogenital disease caused by the fastidious gram-negative bacterium Haemophilus ducreyi, is one of the least understood sexually transmitted diseases. The association between genital ulcer disease and transmission of the human immunodeficiency virus makes control and prevention of chancroid a public health concern. This research project involves investigation of two sets of H. ducreyi proteins which have in common the fact that they are released into culture supernatant fluid and have the potential to affect the host-parasite interaction in chancroid. The first set of protein are two very large macromolecules, designated LspA1 and LspA2, which contribute to the ability of H. ducreyi to resist killing by normal human serum. In addition, LspA1 affects virulence expression by H. ducreyi in an animal model independent of its involvement in serum resistance. The other set of proteins is encoded by the cdtABC gene cluster and comprise the cytolethal distending toxin (CDT) which has cytotoxic activity in vitro against human epithelial cells, human keratinocytes, and human T-cells. In the first Specific Aim, the PI will determine the molecular basis for how LspA1 affects virulence expression by H. ducreyi an how both LspA1 and LspA2 affect serum resistance of this pathogen. The second Specific Aim will involve characterization of the mature LspA1 and LspA2 proteins and investigation into whether the LspB protein effects their release from the H. ducreyi cell. In the third Specific Aim, the PI will identify the H. ducreyi gene product(s) which regulates expression of the LspA1, LspA2 and the LspB proteins. The fourth Specific aim entails the determination of the composition of the H. ducreyi CDT. The fifth and final Specific Aim will evaluate the LspA1, LspA2 and CDT proteins for their ability to induce immunity against H. ducreyi in an animal model.